1. Field of the Invention
The present invention is directed, in general, to wireless communication systems and, more specifically, to a Single-Carrier Frequency Division Multiple Access (SC-FDMA) communication system and is further considered in the development of the 3rd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE).
2. Description of the Art
In particular, the present invention is directed to the transmission of positive or negative acknowledgement signals (ACKs or NACKs, respectively) and Channel Quality Indicator (CQI) signals over the same transmission time interval in an SC-FDMA communication system.
Several types of signals should be supported for the proper functionality of the communication system. In addition to data signals, which convey the information content of the communication, control signals also need to be transmitted from User Equipments (UEs) to their serving Base Station (BS or Node B) in the UpLink (UL) of the communication system and from the serving Node B to the UEs in the DownLink (DL) of the communication system in order to enable the proper transmission of data signals.
The present invention considers the UL communication and assumes that the transmission of signals carrying the data content information from UEs is through a Physical Uplink Shared CHannel (PUSCH) while, in the absence of data information, the transmission of control signals from the UEs is through the Physical Uplink Control CHannel (PUCCH). A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a wireless modem card, etc. A Node B is generally a fixed station and may also be called a Base Transceiver System (BTS), an access point, or some other terminology.
The ACK/NACK is a control signal associated with the application of Hybrid Automatic Repeat reQuest (HARQ) and is in response to the correct or incorrect, respectively, data packet reception in the DL of the communication system (also known as HARQ-ACK). A data packet is retransmitted after the reception of a NACK and a new data packet may be transmitted after the reception of an ACK.
The CQI is another control signal that provides information to the serving Node B about the channel conditions, such as the Signal-to-Interference and Noise Ratio (SINR), experienced in portions of or over the entire DL operating bandwidth. The present invention further considers that the ACK/NACK and CQI transmissions are in the absence of any data transmission from a reference UE.
The UEs are assumed to transmit data or control signals over a Transmission Time Interval (TTI), which in an exemplary embodiment of the present invention corresponds to a sub-frame.
FIG. 1 illustrates a block diagram of a sub-frame structure 110 assumed in an exemplary embodiment of the present invention. The sub-frame includes two slots. A first slot 120 further includes seven symbols used for the transmission of data and/or control signals. Each symbol 130 further includes a Cyclic Prefix (CP) in order to mitigate interference due to channel propagation effects. The signal transmission in one slot may be in the same part or it may be at a different part of the operating bandwidth than the signal transmission in the other slot. In addition to symbols carrying data or control information, some symbols may be used for the transmission of Reference Signals (RS), also known as pilots, used to provide channel estimation and enable coherent demodulation of the received signal. It is also possible for the TTI to include only one slot or more than one sub-frames.
The transmission BandWidth (BW) is assumed to include frequency resource units that will be referred to herein as Resource Blocks (RBs). An exemplary embodiment of the present invention assumes that each RB includes 12 sub-carriers, and that UEs are allocated a multiple N of consecutive RBs 140 for PUSCH transmission and 1 RB for PUCCH transmission. Nevertheless, it should be noted that the above values are only illustrative and should be not restrictive to the described embodiments of the invention.
FIG. 2 illustrates an exemplary structure for a CQI transmission during one slot 210 in a SC-FDMA communication system. The CQI information bits 220, through modulators 230, modulate a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence 240, for example with QPSK or 16QAM modulation, which is then transmitted by the UE, after performing an Inverse Fast Fourier Transform (IFFT) operation as it is further subsequently described. In addition to the CQI, RS is transmitted to enable coherent demodulation at the Node B receiver of the CQI signal. In an exemplary embodiment, the second and sixth SC-FDMA symbols in each slot carry the RS transmission 250.
As mentioned above, the CQI and RS signals are assumed to be constructed from CAZAC sequences. An example of such sequences is given by the following Equation (1):
                                          c            k                    ⁡                      (            n            )                          =                              exp            ⁡                          [                                                                    j                    ⁢                                                                                  ⁢                    2                    ⁢                    π                    ⁢                                                                                  ⁢                    k                                    L                                ⁢                                  (                                      n                    +                                          n                      ⁢                                                                        n                          +                          1                                                2                                                                              )                                            ]                                .                                    (        1        )            
In Equation (1), L is a length of the CAZAC sequence, n is an index of an element of the sequence n={0, 1, 2 . . . , L−1}, and k is an index of the sequence itself. For a given length L, there are L−1 distinct sequences, if L is prime. Therefore, an entire family of sequences is defined as k ranges in {1, 2 . . . , L−1}. However, it should be noted that the CAZAC sequences used for the CQI and RS generation need not be generated using the exact above expression as will be further discussed below.
For CAZAC sequences of prime length L, the number of sequences is L−1. As the RBs are assumed to include an even number of sub-carriers, with 1 RB including 12 sub-carriers, the sequences used to transmit the ACK/NACK and RS can be generated, in the frequency or time domain, by either truncating a longer prime length (such as length 13) CAZAC sequence or by extending a shorter prime length (such as length 11) CAZAC sequence by repeating its first element(s) at the end (cyclic extension), although the resulting sequences do not fulfill the definition of a CAZAC sequence. Alternatively, the CAZAC sequences can be directly generated through a computer search for sequences satisfying the CAZAC properties.
An exemplary block diagram for a transmission of a CAZAC sequence through SC-FDMA signaling in the time domain is illustrated in FIG. 3. The structure illustrated in FIG. 3 can be used, for example, for the CQI transmission in the PUCCH.
Referring to FIG. 3, the CAZAC sequence 310 is generated through one of the previously described methods (modulated for transmission of CQI bits, un-modulated for RS transmission), and is then cyclically shifted 320 as will be subsequently described. The Discrete Fourier Transform (DFT) of the resulting sequence is then obtained 330, the sub-carriers 340 corresponding to the assigned transmission bandwidth are selected 350, the IFFT is performed 360, and finally the cyclic prefix (CP) 370 and filtering 380 are applied to the transmitted signal. Zero padding is assumed to be inserted by the reference UE in sub-carriers used for the signal transmission by another UE and in guard sub-carriers (not shown).
Moreover, for brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not illustrated in FIG. 3. Similarly, the encoding process and the modulation process for CQI bits, which are well known in the art, such as block coding and QPSK modulation, are also omitted for brevity.
At the receiver, the inverse (complementary) transmitter functions are performed. This is conceptually illustrated in FIG. 4, in which the reverse operations of those in FIG. 3 apply.
As it is known in the art (although not shown for brevity), an antenna receives the radio-frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the digital received signal 410 passes through a time windowing unit 420 and the CP is removed 430. Subsequently, the receiver unit applies an FFT 440, selects 450 the sub-carriers 460 used by the transmitter, applies an Inverse DFT (IDFT) 470, de-multiplexes (in time) the RS and CQI signal 480, and after obtaining a channel estimate based on the RS (not shown), extracts the CQI bits 490.
As for the transmitter, well known in the art receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity and they are not material to the invention.
An alternative generation method for the transmitted CAZAC sequence is in the frequency domain, which is illustrated in FIG. 5.
Referring to FIG. 5, the generation of the transmitted CAZAC sequence in the frequency domain follows the same steps as in the time domain with two exceptions. The frequency domain version of the CAZAC sequence is used 510 (that is, the DFT of the CAZAC sequence is pre-computed and not included in the transmission chain) and the cyclic shift 550 is applied after the IFFT 540. The selection 520 of the sub-carriers 530 corresponding to the assigned transmission bandwidth, and the application of cyclic prefix (CP) 560 and filtering 570 to the transmitted signal 580, as well as other conventional functionalities (not shown), are the same as previously described for FIG. 3.
The reverse functions are again performed for the reception of the CAZAC-based sequence transmitted as described in FIG. 5. As is illustrated in FIG. 6, the received signal 610 passes through a time windowing unit 620 and the CP is removed 630. Subsequently, the cyclic shift is restored 640, an FFT 650 is applied, and the transmitted sub-carriers 660 are selected 665. FIG. 6 also illustrates the subsequent correlation 670 with the replica 680 of the CAZAC-based sequence. Finally, the output 690 is obtained, which can then be passed to a channel estimation unit, such as a time-frequency interpolator, in case of a RS, or can be used for detecting the transmitted information, in case the CAZAC-based sequence is modulated by the CQI information bits.
As described above, if the transmitted CAZAC-based sequence illustrated in FIG. 3 or FIG. 5 is not be modulated by any information (data or control), it can then serve as the RS. For CQI transmission, the CAZAC-based sequence is obviously modulated by the CQI information bits (for example, using QPSK modulation). FIG. 3 and FIG. 5 are then modified in a straightforward manner to include the real or complex multiplication of the generated CAZAC sequence with the CQI information symbols. FIG. 2 illustrates such a modulation of a CAZAC sequence.
Different cyclic shifts of the same CAZAC sequence provide orthogonal CAZAC sequences. Therefore, different cyclic shifts of the same CAZAC sequence can be allocated to different UEs in the same RB for their RS or CQI transmission, and achieve orthogonal UE multiplexing. This principle is illustrated in FIG. 7.
Referring to FIG. 7, in order for the multiple CAZAC sequences 710, 730, 750, and 770 generated correspondingly from multiple cyclic shifts 720, 740, 760, and 780 of the same root CAZAC sequence to be orthogonal, the cyclic shift value Δ 790 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If Ts is the duration of one symbol, the number of cyclic shifts is equal to the mathematical floor of the ratio TS/D. For 12 cyclic shifts and for symbol duration of about 66 microseconds (14 symbols in a 1 millisecond sub-frame), the time separation of consecutive cyclic shifts is about 5.5 microseconds. Alternatively, to provide better protection against multipath propagation, only 6 cyclic shifts may be used providing time separation of about 11 microseconds.
The first exemplary setup of the present invention assumes that the UL slot structure for CQI transmission comprises of 5 CQI and 2 RS symbols in 1 RB in each of the 2 slots of the sub-frame (the structure in one slot is illustrated in FIG. 2, the same or a similar structure is repeated for the second slot). During the first slot of the sub-frame the transmission is towards one end of the operating bandwidth and during the second slot it is typically towards the other end of the operating bandwidth (not necessarily the first or last RB of the operating bandwidth, respectively). Nevertheless, transmission may be only in one slot.
Occasionally, it is likely that a UE needs to transmit an ACK/NACK signal, in response to a previously received data packet in the DL of the communication system during the same sub-frame the UE has its CQI transmission in the PUCCH (i.e., the UE has no information data to transmit in the PUSCH). To accomplish this transmission without affecting the multiplexing capacity of ACK/NACK and CQI signals, the prior art considers that the UE suspends the CQI transmission in one or more symbols in order to transmit the ACK/NACK information. This is illustrated in FIG. 8.
Comparing to an equivalent structure of FIG. 2 which does not have any ACK/NACK transmission in the slot 810, one SC-FDMA symbol used for CQI transmission is being replaced by an ACK/NACK transmission 820 leading to a reduction in the number of CQI transmission symbols 830, 835 while the number of RS transmission symbols 840 remains unchanged. Similarly to the CQI bits, the ACK/NACK bits modulate 850 a CAZAC-based sequence 860. The same concept may apply on both slots of a sub-frame if the transmission is over the sub-frame. Therefore, as is the case for the CQI and RS transmission, ACK/NACK is also transmitted by modulating a CAZAC sequence.
When multiplexing ACK/NACK transmission on the same slot or sub-frame as the CQI transmission as illustrated in FIG. 8, a smaller number of CQI information bits should be transmitted in order to avoid decreasing the reliability of the CQI transmission. Alternatively, in order to transmit the same number of CQI information bits a higher code rate should be used, thereby leading to reduced reliability for the received codeword and different coding and decoding processes (depending on whether or not ACK/NACK is also transmitted).
In addition to degrading the CQI reception reliability or reducing the CQI transmission payload, the structure illustrated in FIG. 8 severely limits the ACK/NACK performance as only one symbol per slot is used for ACK/NACK instead of multiple symbols per slot as for example when only ACK/NACK bits (no CQI bits) are transmitted in a slot (except in symbols having RS transmission, if any).
Therefore, puncturing CQI symbols to insert ACK/NACK symbols in the PUCCH is associated with significant performance disadvantages for the transmission of both of these control signals.
Therefore, there is a need to multiplex ACK/NACK information bits in a CQI transmission sub-frame without penalizing the CQI or ACK/NACK performance.
There is another need to multiplex transmission of ACK/NACK information bits in a CQI transmission sub-frame without reducing the number of CQI information bits.
Finally, there is another need to multiplex transmission of ACK/NACK information bits in a CQI transmission sub-frame without substantially changing the transmitter or receiver structure relative to the case of individual transmission for either of these two control signals.